Use GetWindowRect, CombineRgn WinAPI functions, like this example:
var Wnd: HWnd; Region1, Region2: HRgn; Rect: TRect; begin Wnd:=Application.MainForm.Handle; GetWindowRect(Wnd,Rect); Region1:=CreateRectRgn(0,0,Rect.Right-Rect.Left,Rect.Bottom-Rect.Top); Region2:=CreateEllipticRgn(10,30,150,120); CombineRgn(Region1,Region1,Region2,RGN_DIFF); SetWindowRgn(Wnd,Region1,True); end;